Author Topic: Multiple DACs for one single output  (Read 245 times)

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Offline elekorsi

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Multiple DACs for one single output
« on: October 29, 2020, 04:31:26 pm »
Hello everyone,

I couldn't decide either to put this here or into repair section, but this question is not so much about the repair, but to enlighten me with it's operation. I really don't know that much about analogue tech, so this makes me a beginner  :)

I recently took this board out of an old textile warping machine after the customer was experiencing some problems on the machine. The main suspect was this particular board.
This is a brake computer for controlling the main brake of the warping machine. It calculates the correct brake force, so the yarns stay at correct tension when the stop is applied. The main input data to the "computer" are two incremental encoders, one on main beam and one the free rolling roller driven by the yarns. The main output is 0-10V to the PWM power controller, which drives the electromagnetic brakes.

While inspecting the board something raised my eyebrows, and i would be really happy if someone could explain it to me. Op amps are all TL071, DACs are AD667JN.
I made a sketch of the analog part that drives the main output for the brake. Please excuse my CAD, it makes schematics look like they would be hand drawn  :)

And my main question is: Why THREE DACs controlled from ONE single processor to drive ONE output via the summing OP amps?
 

Online rstofer

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Re: Multiple DACs for one single output
« Reply #1 on: October 29, 2020, 10:00:55 pm »
Easy!  The top two op amps are integrators.  The one on the left deals with acceleration and provides a velocity input (integral of acceleration) to the 2d op amp which integrates the velocity inputs to create a displacement value (integral of velocity) which goes to the op amp on the right, in the middle.

I think if you consider the analog bit to be solving a second order ordinary differential equation of motion, you will see where the signals come from and why the circuit is designed the way it is.

Pretty clever motion control!

ETA:  The DACs could be providing a signal from a measurement or a constant for the equation.

« Last Edit: October 29, 2020, 10:05:04 pm by rstofer »
 

Offline elekorsi

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Re: Multiple DACs for one single output
« Reply #2 on: October 31, 2020, 06:30:52 am »
But wouldn't that be normaly solvable in the software and then converted with one DAC instead with three of them?
 

Offline MosherIV

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Re: Multiple DACs for one single output
« Reply #3 on: October 31, 2020, 09:38:47 am »
Depends. The main problem is processing power verses time.
With older, especially 8 bit, processors their processing power may not be enough to perform the calculation in the short time period needed.
Doing the calculation with 'analogue computers' is a clever solution to lack of processing power.
 

Online rstofer

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Re: Multiple DACs for one single output
« Reply #4 on: October 31, 2020, 04:38:27 pm »
My latest interest area in electronics IS analog computing.  Sometimes with hardware (I have 3 analog computers) and sometimes with MATLAB-Simulink.

In the case of this board, I expect one of the DACs is some kind of fixed offset (null) value and the other DACs to have some time varying input and it may be all the CPU can do just to provide those signals.  This board is the first time I have seen an actual analog computer driven by a uC to solve a real-world problem.

Floating point was SLOW on 2 MHz 8080s and not much faster on 6 MHz Z80s.  I didn't look at the board close enough to identify the uC but it probably isn't one of the fast ARMs.
 


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